The present invention relates to a vehicle steering apparatus having a variable transmission ratio device.
As a typical vehicle steering apparatus, there is a type that changes steering characteristics of a vehicle by switching power assistance characteristics or variable transmission ratio (variable gear ratio) characteristics of the vehicle (see, for example, Japanese Laid-Open Patent Publications Nos. 2002-225701 and 5-69848). For example, a plurality of types of variable transmission ratio characteristics are set in correspondence with different traveling states and controlled in accordance with different “modes” (such as normal, sport, and parking modes). The modes are selected and switched either through operation of the driver or automatically. In this manner, the steering characteristics are optimized in correspondence with the current traveling state.
However, in switching of the variable transmission ratio characteristics, the steered angle representing the control target (the target steered angle) may be greatly altered through change of the transmission ratio (the gear ratio). That is, for example, if the gear ratio is changed from 30% to 50% when the steering wheel turning angle is maintained at a constant value of 300°, the target steered angle is greatly altered from 90° to 150°. This causes the variable transmission ratio device to rapidly operate to adjust the actual steered angle in correspondence with the change of the target steered angle, or to quickly change the steered angle. Such rapid operation of the variable transmission ratio device, or quick change of the steered angle, may influence the behavior or steering feel of the vehicle.
To solve this problem, typically, a difference in the target steered angle caused through switching of the variable transmission ratio characteristics is temporarily stored in a memory. When the deviation between the target steered angle and the actual steered angle is computed, small portions of the difference are gradually combined with the target steered angle. In this manner, the target steered angle is offset, and the difference in the target steered angle gradually is reduced to almost zero. This prevents the above-described problem.
However, if the variable transmission ratio characteristics are switched when steering is performed at a great operating angle, the difference in the target steered angle is also great. This limits the effect on switching of the variable transmission ratio characteristics regardless of the above-described procedure. Further, if the steering wheel is returned to the neutral position before the aforementioned difference is completely canceled, the difference that has not been cancelled causes the neutral position of the steering wheel to be offset from the neutral position of the steered wheels. This problem has yet to be solved and, in this regard, the vehicle steering apparatus must be further improved.